This is difficult problem that few students can work it out.A. soB. so aC. suchD. such a

问题补充：

This is difficult problem that few students can work it out.

A. so

B. so a

C. such

D. such a

答案：

A. so

B. so a

C. such

(答案→)D. such a

解析：so或such……that表示如此……以致的意思，so后跟形容词或副词，而such后跟名词，可数名词单数需加不定冠词a或an，本题的problem是可数名词单数，故填such a，故本题选D。